US4882739A - Method for adjusting clocks of multiple data processors to a common time base - Google Patents
Method for adjusting clocks of multiple data processors to a common time base Download PDFInfo
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- US4882739A US4882739A US07/148,493 US14849388A US4882739A US 4882739 A US4882739 A US 4882739A US 14849388 A US14849388 A US 14849388A US 4882739 A US4882739 A US 4882739A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J3/00—Time-division multiplex systems
- H04J3/02—Details
- H04J3/06—Synchronising arrangements
- H04J3/0635—Clock or time synchronisation in a network
- H04J3/0682—Clock or time synchronisation in a network by delay compensation, e.g. by compensation of propagation delay or variations thereof, by ranging
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- This invention relates to a method for synchronizing at least one slave clock to a master clock, and is particularly applicable to distributed data acquisition and/or data processing systems.
- control and data streams are often distributed among many processors.
- current systems depend on:
- Shared hardware for their synchronization typically a common clock and reset line, requiring a direct connection between the common clock and each of the stations to be synchronized to the common clock; or
- each station In a systems having multiple stations it is desirable for each station to have its own clock, so that the station can continue operating even if synchronization with the common clock is temporarily lost.
- clocks may operate at slightly different frequencies, further compounding the time resolution/synchronization problem.
- an object of the present invention is to provide a method for adjusting the clocks of multiple stations (which can but need not necessarily be data acquisition/processing stations) to a common time base.
- Another object of the invention is to provide such a method which is capable of minimizing the adverse effects of transmission time.
- Still another object of the invention is to provide such a method which is capable of minimizing the effects of variation in clock frequency among the various clocks involved.
- Still another object of the invention is to provide such a method which is capable of minimizing the effects of variation in reference time among the various clocks involved.
- Yet another object of the invention is to meet the aforementioned objectives through the use of standard data communications means and standard computer operating systems.
- a method for synchronizing the frequency of a slave clock to that of a master clock wherein the master clock provides a master clock time signal and the slave clock provides a slave clock time signal.
- the slave clock time signal frequency and reference time values can be set independently.
- a time interval commencement signal is transmitted from the master clock to the slave clock.
- the time interval commencement signal has a value corresponding to the value of the master clock time signal when the time interval commencement signal is transmitted.
- a time interval termination signal is subsequently transmitted from the master clock to the slave clock.
- the time interval termination signal has a value corresponding to the value of the master clock time signal when the time interval termination signal is transmitted.
- the ratio k clkratio of the two clock frequencies is computed as the ratio of (i) the difference between the values of the time interval commencement and time interval termination signals to (ii) the elapsed time between reception of the time interval commencement and time interval termination signals as determined by the slave clock.
- An adjusted slave clock time signal is then generated at the slave clock, the adjusted slave clock time signal having a value which increases with time by an amount proportional to the product of the number of periodic slave clock time increment signals with k clkratio .
- a method for synchronizing the reference time of at least one slave clock to that of a master clock The master clock provides a master clock time signal and the slave clock provides a slave clock time signal and periodic slave clock time increment signals.
- a first reference time signal is transmitted from the slave clock to the master clock, said signal having a value corresponding to the value of the slave clock time signal when the first reference time signal is transmitted.
- a second reference time signal is subsequently transmitted from the master clock to the slave clock, said signal having a value corresponding to the value of the master clock time signal when the first reference time signal was received by the master clock.
- a third reference time signal is transmitted from the master clock to the slave clock, said signal having a value corresponding to the value of the master clock time signal when the third reference time signal is transmitted.
- the reference time is computed by:
- the slave clock is then adjusted by an amount equal to the reference time value so determined.
- FIG. 1 is a diagram illustrating principles of the present invention involved in the determination of the difference in frequency between the slave clock at one of a number of data gathering and/or processing stations and the master clock at a central station;
- FIG. 2 is a diagram illustrating principles of the present invention involved in the determination of the difference between the reference time of a slave clock at one of a number of data gathering and/or processing stations and the reference time of a master clock at the central station, as well as the transmission time betwen said clocks;
- FIG. 3 is a diagram of a set of three data acquisition radar satellite stations communicating with a common ground station;
- FIG. 4 is a block diagram of the data acquisition and synchronization circuitry of one of said satellite stations
- FIG. 5 is a graph showing the probability distribution of the signal transmission time between one of the satellite stations and the ground station;
- FIG. 6 is a flow chart showing the synchronization signal processing steps which take place at said one satellite station and at the ground station for the determination of the ratio (k clkratio ) between the frequency of the master clock at the ground station and the frequency of the slave clock at the satellite station;
- FIG. 7 is a timing diagram for the determination of the frequency ratio k clkratio by said one satellite station
- FIG. 8 is a flow chart showing the synchronization signal processing steps which take place at said one satellite station and at the ground station for the determination of the reference time (t ref ) by which the slave clock of the satellite station is to be adjusted so as to correspond to the time of the master clock at the ground station;
- FIG. 9 is a timing diagram for the determination of the reference time t ref at said one satellite station.
- FIG. 10 is a flow chart showing the data acquisition, data processing and time synchronization steps involved in the coordinated processing of radar signals from an object by each of the satellite stations.
- FIG. 11 is a flow chart showing synchronization signal processing in each satellite station and the ground station, to provide an optional feature of the invention wherein the master clock at the ground station is set to a reference time which corresponds to the average of the reference times of the various stations, and is operated at an effective frequency corresponding to the average of the frequencies of the various clocks in the system.
- each clock normally operates by counting clock pulses generated by a local oscillator, and by incrementing a starting time to which the clock is initially set, in accordance with the number of pulses counted.
- clock refers to an arrangement which includes a local oscillator for generating periodic clock signals, a counter for counting the clock signals to generate an initial digital time signal; and an associated processor for converting the initial digital time signal generated by the counter to a desired time signal in accordance with desired reference time and/or frequency standards.
- the local oscillator frequency is not adjusted to match a desired frequency standard. Rather, the relationship between the initial digital time signal and the desired time signal is changed accordingly.
- the desired time signal is changed to correspond to a desired reference time standard.
- the present invention provides methods for determining the transmission time between clocks and the frequency ratios and differences in reference time between clocks with a very high degree of accuracy, so that the clocks can be synchronized to an extent not heretofore possible.
- a master clock is situated at a central data gathering station, and slave clocks are situated at one or more (functionally or spatially) remote stations.
- Time signals are exchanged between the master clock at the central station and each slave clock at the corresponding remote station. From these signals (i) the transmission time between the central station and the corresponding remote station is (directly or indirectly) determined, (ii) the ratio between the frequencies of the central station master clock and the corresponding remote station slave clock is determined, (iii) the difference between the reference times of the central station master clock and the corresponding remote station slave clock is determined.
- the output of the slave clock is adjusted by a time increment equal to the reference time, and incremented at a rate adjusted by the ratio between the frequencies of the master and slave clocks so that the frequency of the slave clock is then synchronized to that of the master clock.
- the time interval over which the ratio of the frequencies of the master and slave clocks is determined is preferably as long as is practicable, for greatest accuracy.
- the number of signals exchanged between the master and slave clocks should be as great as possible. These signals are averaged to provide improved accuracy.
- the reference time value is used to adjust the slave clock to a value accurately corresponding to the time kept by the master clock; and the clock ratio value is used to insure that the slave clock is incremented at a rate corresponding to the frequency of the oscillator in the master clock.
- the master clock can be synchronized to a reference time which is the average of the reference times of the various clocks, and/or to a frequency which is the average of the frequencies of the oscillators in the various clocks.
- a time signal sequence is initiated by, for example, the master clock transmitting to the slave clock a first time signal (which may be a time interval commencement signal) having a value MT 0 corresponding to the master clock time at which the signal is sent, e.g. 2:00:00.00 p.m. (i.e., two hours after the master clock starting time of 0:00:00.00 as measured by the master clock).
- a first time signal which may be a time interval commencement signal having a value MT 0 corresponding to the master clock time at which the signal is sent, e.g. 2:00:00.00 p.m. (i.e., two hours after the master clock starting time of 0:00:00.00 as measured by the master clock).
- the slave clock would receive the first signal (with value MT 0 ) at a time ST 0 of 2:00:00.14 p.m. as determined by the slave clock.
- a second time signal (which may be a time interval termination signal) MT 1 with master clock value of 3:00:00.00 p.m. is transmitted to the slave clock.
- the slave clock would receive the second time signal (with value MT 1 at a time ST 1 of 4:00:00.14 p.m. as determined by the slave clock.
- the clock ratio k clkratio i.e. the ratio of the master clock local oscillator frequency to the slave clock local oscillator frequency, is given by the ratio of the elapsed time between transmission of the first (time interval commencement) and second (time interval termination) signals as measured by the master clock, to the elapsed time between reception of those signals as measured by the slave clock.
- a first time signal ST 0 is transmitted from the slave clock to the master clock.
- the first time signal has a value corresponding to the value of the slave clock time signal at the time when the first time signal is transmitted, i.e. 2:00:00.00 p.m.
- a second time signal MT 0 is subsequently transmitted from the master clock to the slave clock.
- the second time signal has a value corresponding to the value of the master clock time signal when the first time signal was received by the master clock, i.e. 2:00:00.07 p.m.
- a third time signal MT 1 is subsequently transmitted from the master clock to the slave clock.
- the third time signal has a value corresponding to the value of the master clock time signal when the third time signal was transmitted by the master clock, i.e. 3:00:00.00 p.m.
- the slave clock would receive the third signal (with value MT 1 ) at a time ST 1 of 4:00:00.14 p.m. as determined by the slave clock.
- the average of successive measurements of the time signals is used.
- the measurements of k clkratio can be carried out from time to time, but the accuracy of the measurement is determined by the total interval over which the measurements are made.
- t ref and T TR can be carried out from time to time, but measurements based upon multiple time exchanges are preferred for greatest accuracy.
- the slave clock recalculated (virtual clock) time T vc is given by
- n pc is the number of periodic slave clock time increment signals generated.
- the clock system at each (spatially or functionally) remote station models a virtual or "world” clock (e.g. the master clock at the central station) in terms of its own local physical (slave) clock; and uses information gathered from communication with the master clock to closely approximate the model's parameters.
- a virtual or "world” clock e.g. the master clock at the central station
- T vc the virtual (master) clock time (i.e. the adjusted slave clock time), is an absolute quantity expressed in terms of the modeling parameter t ref (the reference time of the virtual clock) and k clkratio (the ratio between the frequencies of the virtual (master) clock and the physical (slave) clock) and the physical parameter n pc (the number of ticks or periods which have elapsed on the physical (slave) clock in the remote processor).
- the processor at each station determines the parameters t ref and k clkratio and thus can compute the virtual (master) clock time T vc from n pc , its physical (slave) clock time and vice versa.
- the system depicted in FIG. 3 consists of a ground station 1 and a number of satellites 3a, 3b and 3c.
- the ground station 1 communicates with the satellites via transmissions over bidirectional radio links 2a, 2b and 2c respectively.
- the ground station has a radio transmittion/reception antenna 6 while the satellites have radio transmission/reception antennae 7a, 7b and 7c respectively.
- Each satellite contains a radar system (4a, 4b4c).
- the ground station 1 sends a message to each of the satellites telling them what time to send a radar pulse toward an area where it is desired to detect an object.
- the radar pulses must be sent from all satellites at the same time, or at times coordinated so that a desired phased array effect can be achieved.
- each satellite Upon sending its radar pulse, each satellite samples the amplitude of the incoming signals received at its radar dish and determines the (adjusted (to master clock time) slave clock time) when the peak (maximum amplitude) signal occurred.
- the peak occurrence time along with the sampled data is stored in the memory of the satellite processor. This occurrence time and sampled data is then transmitted to the ground station.
- the ground station compares the received peak (time and amplitude) occurrence data from the set of satellites and determines if an object has been detected. If so, the satellites are instructed to send their complete sets of data samples for further analysis by the ground station.
- the satellites For the satellites to send the radar pulses at the same time (or at coordinated times) and for the ground station to compare the data streams from the group of satellites, the satellites must each measure time by the same standard, i.e. a common virtual clock from which to temporally reference their actions and data.
- ground station equipment Since the ground station equipment is under fewer constraints than the satellites, it makes sense to provide it with a very accurate absolute or "master” clock and use it as the "virtual" clock to which all the satellites must time-synchronize.
- Each satellite then computes the model parameters t ref and k clkratio and adjusts or corrects its "slave" clock time values such that the data sent to the ground stations is as though the satellites used the actual ground station master clock as their time base for the data acquisition. Additionally, each satellite synchronizes all its actions relative to the ground station master clock.
- FIG. 4 shows a block diagram of the data acquisition and synchronization circuitry of one of said satellite stations.
- the data processor 8 controls the system and performs the computations associated with the time corrections.
- the random access memory or RAM 9 contains the timing variables (t ref , k clkratio ), the raw collected data, and the time-synchronized collected data.
- the read only memory or ROM 10 contains the programs associated with system control and time-synchronization.
- the receiver/transmitter 11 communicates with the ground station.
- the timer 12 is a simple counter driven by the local oscillator 13.
- the oscillator 13 provides the driving frequency for the timer 12, which counts pulses derived from the oscillator.
- the frequency of the oscillator cannot be set exactly and thus will vary slightly among the satellites.
- the controller 14 receives commands from the data processor 8 via the common signal bus 15 and sends out radar pulses via the radar dish 4a.
- Radar signals received by the radar dish 4a are coupled to the signal processor 16, which transforms them to levels acceptable for the analog-to-digital (A/D) converter 17.
- A/D analog-to-digital
- the A/D converter 17 receives the analog data from the signal processor 16 and converts it to a stream of digital data for the data processor 8.
- Time values are bidirectionally transmitted between the ground station and the satellites, as previously described.
- the transmission time T TR is defined as the time required for the time message to be generated, transmitted, received and acted upon.
- the uncertainty of the time period required for the transmission of time information can be reduced by directly linking the timer 12 to the receiver/transmitter 11, as shown by the dashed line in FIG. 4.
- T TR can be modeled as an average value T TRavg with a limited variation T TRvar . (See FIG. 5). That is,
- T TRavg If deviations in T TR from T TRavg are essentially independent, then averaging successive observations of T TR should improve the determination of T TRavg by the square root of the number of observations.
- the techniques of the present invention make extensive use of this averaging to increase the system performance of the system beyond the limits imposed by a single determination of T TRavg .
- the present invention utilizes the time of transmission of a time signal as measured by the transmitter's clock and the time of reception of the same time signal as measured by the receiver's clock.
- the absolute time difference between transmission and reception of a time signal is defined as the transmission time.
- Equation (5) must be modified to account for the transmission time when equating transmission and reception time values.
- the technique employed for the determination by a satellite of the difference between its (slave) clock frequency and the ground station's (master) clock frequency is to measure the same elapsed time interval with the ground station clock and the satellite clock.
- the measured value of elapsed time is directly proportional to the measuring clock's frequency.
- the ratio of the measurements of elapsed time provides a value for k clkratio .
- the ground station At time MT 0 the ground station records the time on its clock (Step 1). The ground station then sends a message to the satellite (Step 2) containing the time value MT 0 . Te satellite receives the message (Step 3) and reads the time ST 0 on its clock (Step 4).
- the ground station At time MT 1 the ground station records the time on its clock (Step 5). The ground station then sends a message to the satellite (Step 6) containing the time value MT 1 . The satellite receives the message (Step 7) and reads the time ST 1 on its clock (Step 8).
- the satellite now has the values ST 0 , ST 1 , MT 0 and MT 1 .
- Equation (7) Substituting the pairs of time values MT 0 , ST 0 ) and (MT 1 , ST 1 ) into Equation (7) yields:
- t ref terms cancel because t ref is a constant defining the relationship between the starting times of the two clocks.
- Equation (6) Substituting Equation (6) into Equation (11) yields: ##EQU1##
- the master clock transmits subsequent time interval termination signals.
- the slave clock uses the most recently received termination signal (transmitted at time MT n as measured by the master clock and received at time ST n as measured by the slave clock) to compute a more accurate estimate of k clkratio using Equation (16):
- the reference time determination method of the present invention yields best results when the average transmission time from the ground station to the satellite is equal to the average transmission time from the satellite to the ground station, as is normally the case; and when the technique described in this application for determining the relationship between the frequencies of the master and slave clocks is also employed.
- the satellite At time ST 0 the satellite records the time on its (slave) clock (Step 1). The satellite then sends a message to the ground station (Step 2) requesting the ground station to read and return the value on its (master) clock. The ground station receives the message (Step 3), reads the time MT 0 on its (master) clock (Step 4), and sends this time to the satellite (Step 5). The satellite receives the time and records it as MT 0 (Step 6).
- the ground station reads the time MT 1 on its (master) clock (Step 7), and sends this time to the satellite (Step 8).
- the satellite receives the time and records it as MT 1 (Step 9).
- the satellite then reads its (slave) clock and records the time the message was received (Step 10) as ST 1 .
- the satellite now has four pieces of information, viz. ST 0 , MT 0 , ST 1 , and MT 1 .
- t ref can be computed from a set of message exchanges between the ground station 1 and the satellite (FIG. 8, Step 11 ).
- the transmission time T TR can also be computed from a set of message exchanges between the ground station and the satellite.
- Such a set of exchanges also provides an alternate method of computing the value of k clkratio . That is, solving Equation (22) for k clkratio yields: ##EQU3##
- Equation (23) [ ⁇ 2 * T TR /(ST 1 -ST 0 )] is larger than the error term of Equation (13) [ ⁇ 2 * T TRvar /(ST 1 -ST 0 )], both approach zero as the time interval approaches infinity.
- the slave clock may transmit a number of master clock read messages to the master clock, each message causing the master clock to read and accumulate the value of the master clock output at the time MT A that the corresponding message is received. At the same time, the slave clock reads and accumulates the value of its output at the time ST A that each corresponding master clock read message is transmitted.
- the master clock transmits a number of slave clock read messages to the slave clock, each such message causing the slave clock to read and accumulate the value of the slave clock output at the time ST b that the corresponding message is received.
- the master clock reads and accumulates the value of its output at the time MT B that each corresponding slave clock read message is transmitted.
- the number n b of such messages need not necessarily be equal to the number n a of master clock read messages transmitted by the slave clock to the master clock.
- ⁇ MT A is the sum of the master clock times of reception of the n a master clock read messages transmitted by the slave clock to the master clock
- ⁇ T TRA is the sum of the transmission times of n a master clock read messages
- K is the ratio k clkratio of the master clock frequency to the slave clock frequency
- ⁇ ST A is the sum of the slave clock times of transmission of the n a master clock read messages
- ⁇ t ref is the sum of n a corresponding values of t ref .
- Equation (25) Substituting Equation (25) into Equation (24):
- WS m and WS s represent weighted sums of the transmission and reception times being accumulated by the master and slave clocks respectively.
- the master clock After a desired number n a of transmissions of master clock read messages and a desired number n b of transmissions of slave clock read messages, the master clock sends the slave clock the accumulated value WS m .
- the slave clock then computes the value of t ref as follows:
- Equation (29) Subtracting Equation (29) from Equation (28) yields: ##EQU10## which reduces to
- WD m and WS s are weighted differences of the transmission and reception times being accumulated by the master and slave clocks.
- the master clock After a desired number n a of transmissions of master clock read messages and a desired number n b of transmissions of slave clock read messages, the master clock sends the slave clock the accumulated value WD m .
- the slave clock then computes the value of T TR as follows:
- the ground station 1 sends a message to each of the satellites 3a, 3b, 3c specifying the (ground station master clock) time to emit the radar pulse (Step 1) and the duration of each of the time intervals thereafter at which samples of radar return signals are to be taken.
- Each satellite receives the message from the ground station (Step 2) and waits until its (slave) clock reaches the specified pulse emission time (Step 3).
- Step 3 When the specified emission time is reached, a pulse is emitted by each of the radar dishes 4a, 4b and 4c (Step 4).
- Each satellite then initializes a number of data collection variables (Steps 5, 6, 7). To begin sampling the data immediately, the satellite sets the first sampling time to the time the pulse was emitted (Step 8).
- Each satellite then waits until its (frequency adjusted slave) clock reaches the first specified sampling time, i.e. at the expiration of the previously specified interval time at which samples are to be taken (Step 9).
- the satellite reads a data sample from its radar dish 4a, 4b or 4c via the A/D converter 17 (Step 10).
- the satellite repeats this process, comparing each data sample to the previously stored (maximum) data sample (Step 11). If the new sample is greater than the previously stored maximum, the satellite updates the recorded maximum value (Step 12) and the time of arrival of the new maximum value (Step 13). The satellite then computes the time of arrival of the next data sample (Step 14), increments the number of data samples collected (Step 15), and tests if all the desired samples have been collected (Step 16).
- the satellite sends the maximum amplitude radar signal receipt time to the ground station (Step 17).
- the ground station receives the maximum amplitude radar signal receipt time for each satellite (Step 18), compares the samples from all satellites, and decides if a significant event was detected (Step 19).
- Step 1 If no event was detected, the process repeats when the ground station 1 requests another radar pulse to be emitted (Step 1).
- the ground station requests that the satellites transmit their data streams to the ground station for analysis (Step 20). Each satellite receives the request (Step 21) and sends the data to the ground station (Step 22), where it is received (Step 23) and processed (Step 24).
- the total process repeats when the ground station sends the satellites a request for another radar pulse to be emitted (Step 1).
- the virtual (master) clock reference from the average of the reference times of all clocks in the system; and to establish the virtual (master) clock frequency as the average of the frequencies of all clocks in the system.
- an average of the parameters can be computed and used to determine the new virtual (master) clock parameters, utilizing the method depicted in FIG. 11.
- the satellites send their parameters t ref and k clkratio to the ground station (Step 1).
- the ground station receives the time parameters (Step 2) and computes the correction factor (Step 3) for k cllkratio such that the virtual clock frequency will be the average of all the clock frequencies in the system, utilizing Equation 47. ##EQU11##
- the ground station then computes the correction factor (Step 4) for t ref such that the virtual (master) clock reference time will be the average of the reference times of all clocks in the system, utilizing Equation 48. ##EQU12##
- the ground station then corrects its clock frequency parameter by applying the average values of k clkratio (Step 5/Equation 49); and corrects its reference time parameter by applying the average of the reference times (Step 6/Equation 50).
- the ground station then transmits the time parameter correction values to each of the satellites (Step 7). These signals are received by the satellites (Step 8) and the frequency and reference time parameters of the satellite (slave) clocks are corrected (Steps 9, 10).
- the model of time utilized in the method described in this application makes a number of assumptions which are normally true, including: a linear relationship between the variables, a stable oscillator driving the clocks, and a constant average transmission time T TRavg .
- the satellites can plot the data used in the time correction algorithm and search for patterns. If patterns are found, e.g. predictable long term fluctuations in the oscillator frequency, they can be corrected for by a more sophisticated model of time using known curve fitting techniques. Similarly, the system can use information about the clocks, their operation and their interrelationship in the derivation of the time parameters.
- auxiliary slave clock can communicate with an intermediate slave clock which in turn communicates with the master clock.
- a primary clock ratio of the frequency of the master clock to the frequency of the intermediate slave clock is determined as previously described; and a primary reference time equal to the difference between the master and intermediate slave clocks is also determined as previously described.
- a secondary clock ratio of the frequency of the intermediate slave clock to the frequency of the auxiliary slave clock is determined as previously described; and a secondary reference time equal to the difference between the intermediate and auxiliary slave clocks is also determined as previously described.
- the auxiliary slave clock then is synchronized to the master clock utilizing a composite reference time and clock ratio instead of conventional reference time and clock ratio values.
- the composite reference time is equal to the sum of the primary reference time and the seconding reference time multiplied by the primary clock ratio, and the composite clock ratio is equal to the product of the primary and secondary clock ratios.
- the primary clock ratio would be 0.5 and the secondary clock ratio would be 0.333, for a composite clock ratio of 0.16666; and this clock ratio would be used in the manner previously described in this application, to synchronize the auxiliary slave clock to the master clock, just as though the auxiliary clock were a "conventional" slave clock.
- the composite reference time would be 2.00, i.e. 2.00 * 0.5+1.00; and this reference time value would be used in the manner previously described in this application, to synchronize the auxiliary slave clock to the master clock, just as though the auxiliary clock were a "conventional" slave clock.
- the master clock can be synchronized to any slave clock using the same techniques that have already been described. That is, at the master clock the value of the slave clock time signal of a particular slave clock corresponding to a given master clock time value MT can be determined according to the relation
- n pc is the number of increments of the slave clock time signal.
- the master clock could specify the time it wants the slave clock to initiate a particular event (such as the transmission of a radar pulse) in (unadjusted) slave clock time instead of master clock time.
- the reference time at a point other than the starting time of the slave clock is referenced.
- the reference time t ref is the difference between the time values of the master and slave clocks at a particular moment.
- the previously presented equations involving reference time are based upon that moment being the starting time of the slave clock, i.e. when the time value of the slave clock is zero; and as previously described for many applications it is preferred that the determination of t ref correspond to this moment.
- t ref corresponds to the difference between the master and slave clock time signal values at a particular slave clock (or master clock) time (here the slave clock starting time)
- the communications and calculations required to determine this value of t ref may be performed at any desired time.
- t ref be determined as the difference between the master and slave clock time signal values at the starting time of the slave clock.
- the reference time t ref can be determined as said difference at any slave clock time, so long as the slave clock time increments are adjusted for any difference between the master and slave clock frequencies on the basis of the number of slave clock time signal increments between the slave clock time signal and the slave clock time signal value corresponding to the time of determination of the reference time.
- the master or virtual clock time T vc when the slave clock has generated a total of n pc time signal increments from its starting time is given by
- Equation (55) reduces to Equation (5).
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Abstract
Description
k.sub.clkratio =(MT.sub.1 -MT.sub.0)/(ST.sub.1 -ST.sub.0) (1)
t.sub.ref =[(MT.sub.0 +MT.sub.1)-(ST.sub.0 +ST.sub.1) * k.sub.clkratio ]/2 (2)
T.sub.TR =[(MT.sub.0 -MT.sub.1)+(ST.sub.1 -ST.sub.0) * k.sub.clkratio ]/2 (3)
T.sub.vc =t.sub.ref +n.sub.pc * k.sub.clkratio (4)
T.sub.vc =t.sub.ref +n.sub.pc * k.sub.clkratio (5)
T.sub.TR =T.sub.TRavg +T.sub.TRvar (6)
MT=ST * k.sub.clkratio +t.sub.ref -T.sub.TR (7)
MT-T.sub.TR =ST * k.sub.clkratio +t.sub.ref (8)
MT.sub.0 =ST.sub.0 * k.sub.clkratio +t.sub.ref -T.sub.TR (9)
MT.sub.1 =ST.sub.1 * k.sub.clkratio +t.sub.ref -T.sub.TR (10)
(MT.sub.1 -MT.sub.0)=(ST.sub.1 -ST.sub.0) * k.sub.clkratio +t.sub.ref -t.sub.ref -T.sub.TR +T.sub.TR (b 11)
k.sub.clkratio =(MT.sub.1 -MT.sub.0)/(ST.sub.1 -ST.sub.0) (14)
Error.sub.max =±2 * T.sub.Trvar / (ST.sub.1 -ST.sub.0) (15)
k.sub.clkratio =(MT.sub.n -MT.sub.0)/(ST.sub.n -ST.sub.0) (16)
MT.sub.0 -T.sub.TR =t.sub.ref +ST.sub.0 * k.sub.clkratio (b 17)
MT.sub.1 =t.sub.ref +ST.sub.1 * k.sub.clkratio -T.sub.TR (b 18)
MT.sub.0 +MT.sub.1 -T.sub.TR =t.sub.ref +ST.sub.O * k.sub.clkratio +t.sub.ref +ST.sub.1 * k.sub.clkratio -T.sub.TR (19)
t.sub.ref =[(MT.sub.0 +MT.sub.1)-(ST.sub.0 +ST.sub.1) * k.sub.clkratio ]/2 (20)
(MT.sub.0 -MT.sub.1)=(ST.sub.0 -ST.sub.1) * k.sub.clkratio +t.sub.ref -t.sub.ref +T.sub.TR +T.sub.TR (21)
T.sub.TR =[(MT.sub.0 -MT.sub.1)+(ST.sub.1 -ST.sub.0) * k.sub.clkratio ]/2 (22)
ΣMT.sub.A -ΣT.sub.TRA =KΣST.sub.A +Σt.sub.ref (24)
Σt.sub.ref =n.sub.a * t.sub.ref (b 25)
ΣMT.sub.A -ΣT.sub.TRA =KΣST.sub.A +n.sub.a * t.sub.ref (26)
t.sub.ref =[(n.sub.b ΣMT.sub.A 30 n.sub.a ΣMT.sub.B)-K(n.sub.b ΣST.sub.A +n.sub.a ΣST.sub.B)]/2n.sub.a n.sub.b (36)
Let WS.sub.m =n.sub.b ΣMT.sub.A +n.sub.a ΣMT.sub.B (b 37)
WS.sub.s =n.sub.b ΣST.sub.A +n.sub.a 93 ST.sub.B (38)
t.sub.ref =(Ws.sub.m -K * WS.sub.s)/2n.sub.a n.sub.b (39)
T.sub.TRA +T.sub.TRB =[(n.sub.b ΣMT.sub.A -n.sub.a ΣMT.sub.B) +K(n.sub.a ΣST.sub.B -n.sub.b ΣST.sub.A)]/n.sub.a n.sub.b (41)
T.sub.TRA +T.sub.TRB =2* T.sub.TRavg (42)
T.sub.TR =[(n.sub.b ΣMT.sub.A -n.sub.a ΣMT.sub.B)+K(n.sub.a ΣST.sub.B -n.sub.b ΣST.sub.A)]/2n.sub.a n.sub.b (b 43 )
Let WD.sub.m =n.sub.b ΣMT.sub.A -n.sub.a ΣMT.sub.B (44)
WD.sub.s =n.sub.a ΣST.sub.B -n.sub.b ΣST.sub.A (45)
T.sub.TR =(WD.sub.m +K * WD.sub.s)/2n.sub.a n.sub.b (46)
k.sub.clkratio =k.sub.clkratio * k.sub.clkratioCORR (49)
t.sub.ref =t.sub.ref * k.sub.clkratioCORR +t.sub.refCORR (50)
n.sub.pc =(MT-t.sub.ref)/k.sub.clkratio (54)
T.sub.vc =t.sub.ref +n.sub.pc0 +(n.sub.pc -n.sub.pc0) * k.sub.clkratio (55)
Claims (33)
t.sub.ref =[(MT.sub.0 +MT.sub.1)-(ST.sub.0 +ST.sub.1)* k.sub.clkratio ]/2
T.sub.vc =t.sub.ref +n.sub.pc * k.sub.clkratio
t.sub.ref =[(MT.sub.0 +MT.sub.1)-(ST.sub.0 +ST.sub.1)]/2; and
T.sub.vc =t.sub.ref +n.sub.pc
T.sub.vc =t.sub.ref +n.sub.pc * k.sub.clkratio
T.sub.vc =t.sub.ref +n.sub.pc
T.sub.vc =t.sub.ref +n.sub.pc
T.sub.vc =t.sub.ref +n.sub.pc * k.sub.clkratio.
T.sub.vc =t.sub.ref +n.sub.pc * k.sub.clkratio
k.sub.clkratio =(MT.sub.1 -MT.sub.0)/(ST.sub.1 -ST.sub.0); and
k.sub.clkratio =(MT.sub.1 -MT.sub.0)/(ST.sub.1 -ST.sub.0); and
t.sub.ref =[(MT.sub.0 +MT.sub.1)-(ST.sub.0 +ST.sub.1)]/2; and
T.sub.vc =t.sub.ref +n.sub.pc
t.sub.ref =[(MT.sub.0 +MT.sub.1)-(ST.sub.0 +ST.sub.1) * k.sub.clkratio ]/2;
T.sub.vc =t.sub.ref +n.sub.pc * k.sub.clkratio.
k.sub.clkratio =(MT.sub.1 -MT.sub.0)/(ST.sub.1 -ST.sub.0); and
T.sub.TR =[(MT.sub.0 -MT.sub.1)+(ST.sub.1 -ST.sub.0)]/2; and
T.sub.TR =[(MT.sub.0 -MT.sub.1)+(ST.sub.1 -ST.sub.0) * k.sub.clkratio)]/2
T.sub.vc =MT.sub.1 +T.sub.TR -ST.sub.1 +n.sub.pc * k.sub.clkratio.
k.sub.clkratio =(MT.sub.1 -MT.sub.0)/(ST.sub.1 -ST.sub.0); and
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US07/148,493 US4882739A (en) | 1988-01-26 | 1988-01-26 | Method for adjusting clocks of multiple data processors to a common time base |
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Cited By (32)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5124980A (en) * | 1989-03-20 | 1992-06-23 | Maki Gerald G | Synchronous multiport digital 2-way communications network using T1 PCM on a CATV cable |
US5153824A (en) * | 1989-10-17 | 1992-10-06 | Alcatel Cit | High stability clock synchronized on an external synchronization signal |
WO1993007681A1 (en) * | 1991-10-04 | 1993-04-15 | Motorola, Inc. | Simulcast synchronization and equalization system and method therefor |
WO1993007682A1 (en) * | 1991-10-04 | 1993-04-15 | Motorola, Inc. | Simulcast synchronization and equalization system and method therefor |
WO1993011614A1 (en) * | 1991-12-06 | 1993-06-10 | Motorola, Inc. | Technique for measuring channel delay |
GB2265280A (en) * | 1990-12-04 | 1993-09-22 | Roke Manor Research | Wide area nodeless distributed synchronisation |
US5293374A (en) * | 1989-03-29 | 1994-03-08 | Hewlett-Packard Company | Measurement system control using real-time clocks and data buffers |
US5483665A (en) * | 1990-11-13 | 1996-01-09 | Pagemart, Inc. | Simulcast satellite paging system with over lapping paging reception locales |
US5530846A (en) * | 1993-12-29 | 1996-06-25 | International Business Machines Corporation | System for decoupling clock amortization from clock synchronization |
US5715438A (en) * | 1995-07-19 | 1998-02-03 | International Business Machines Corporation | System and method for providing time base adjustment |
US5774377A (en) * | 1991-07-30 | 1998-06-30 | Hewlett-Packard Company | Method and apparatus for monitoring a subsystem within a distributed system for providing an archive of events within a certain time of a trap condition |
US5875320A (en) * | 1997-03-24 | 1999-02-23 | International Business Machines Corporation | System and method for synchronizing plural processor clocks in a multiprocessor system |
US6084934A (en) * | 1997-03-06 | 2000-07-04 | International Business Machines Corporation | Natural throttling of data transfer across asynchronous boundaries |
US6098178A (en) * | 1998-05-22 | 2000-08-01 | The United States Of America As Represented By The Secretary Of The Navy | Time synchronization algorithm for massively parallel processor systems |
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US6324586B1 (en) | 1998-09-17 | 2001-11-27 | Jennifer Wallace | System for synchronizing multiple computers with a common timing reference |
US6351821B1 (en) * | 1998-03-31 | 2002-02-26 | Compaq Computer Corporation | System and method for synchronizing time across a computer cluster |
US20020169869A1 (en) * | 2001-05-08 | 2002-11-14 | Shugart Technology, Inc. | SAN monitor incorporating a GPS receiver |
US6501808B1 (en) * | 1999-04-29 | 2002-12-31 | Northrop Grumman Corporation | Apparatus and method for instantaneous reacquisition in a network system |
US6505149B1 (en) * | 1999-08-02 | 2003-01-07 | International Business Machines Corporation | Method and system for verifying a source-synchronous communication interface of a device |
US20030103486A1 (en) * | 2001-11-30 | 2003-06-05 | Metin Salt | Time synchronization using dynamic thresholds |
US6618815B1 (en) | 2000-02-29 | 2003-09-09 | International Business Machines Corporation | Accurate distributed system time of day |
US6788655B1 (en) * | 2000-04-18 | 2004-09-07 | Sirf Technology, Inc. | Personal communications device with ratio counter |
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US5124980A (en) * | 1989-03-20 | 1992-06-23 | Maki Gerald G | Synchronous multiport digital 2-way communications network using T1 PCM on a CATV cable |
US5293374A (en) * | 1989-03-29 | 1994-03-08 | Hewlett-Packard Company | Measurement system control using real-time clocks and data buffers |
US5153824A (en) * | 1989-10-17 | 1992-10-06 | Alcatel Cit | High stability clock synchronized on an external synchronization signal |
US5483665A (en) * | 1990-11-13 | 1996-01-09 | Pagemart, Inc. | Simulcast satellite paging system with over lapping paging reception locales |
GB2265280B (en) * | 1990-12-04 | 1994-10-19 | Roke Manor Research | Wide area nodeless distributed synchronisation (fine sync. maintenance) |
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US5774377A (en) * | 1991-07-30 | 1998-06-30 | Hewlett-Packard Company | Method and apparatus for monitoring a subsystem within a distributed system for providing an archive of events within a certain time of a trap condition |
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WO1993007682A1 (en) * | 1991-10-04 | 1993-04-15 | Motorola, Inc. | Simulcast synchronization and equalization system and method therefor |
US5261118A (en) * | 1991-10-04 | 1993-11-09 | Motorola, Inc. | Simulcast synchronization and equalization system and method therefor |
US5257404A (en) * | 1991-10-04 | 1993-10-26 | Motorola, Inc. | Simulcast synchronization and equalization system and method therefor |
US5280629A (en) * | 1991-12-06 | 1994-01-18 | Motorola, Inc. | Technique for measuring channel delay |
WO1993011614A1 (en) * | 1991-12-06 | 1993-06-10 | Motorola, Inc. | Technique for measuring channel delay |
US5530846A (en) * | 1993-12-29 | 1996-06-25 | International Business Machines Corporation | System for decoupling clock amortization from clock synchronization |
US5715438A (en) * | 1995-07-19 | 1998-02-03 | International Business Machines Corporation | System and method for providing time base adjustment |
US6084934A (en) * | 1997-03-06 | 2000-07-04 | International Business Machines Corporation | Natural throttling of data transfer across asynchronous boundaries |
US5875320A (en) * | 1997-03-24 | 1999-02-23 | International Business Machines Corporation | System and method for synchronizing plural processor clocks in a multiprocessor system |
US6351821B1 (en) * | 1998-03-31 | 2002-02-26 | Compaq Computer Corporation | System and method for synchronizing time across a computer cluster |
US6138243A (en) * | 1998-05-14 | 2000-10-24 | International Business Machines Corporation | Method and system for keeping time across a multiprocessor platform |
US6098178A (en) * | 1998-05-22 | 2000-08-01 | The United States Of America As Represented By The Secretary Of The Navy | Time synchronization algorithm for massively parallel processor systems |
US6324586B1 (en) | 1998-09-17 | 2001-11-27 | Jennifer Wallace | System for synchronizing multiple computers with a common timing reference |
US6501808B1 (en) * | 1999-04-29 | 2002-12-31 | Northrop Grumman Corporation | Apparatus and method for instantaneous reacquisition in a network system |
US6505149B1 (en) * | 1999-08-02 | 2003-01-07 | International Business Machines Corporation | Method and system for verifying a source-synchronous communication interface of a device |
WO2001022202A1 (en) * | 1999-09-17 | 2001-03-29 | Comuniq Asa | Method for synchronizing clocks in electronic units connected to a multi processor data bus |
US6618815B1 (en) | 2000-02-29 | 2003-09-09 | International Business Machines Corporation | Accurate distributed system time of day |
US6788655B1 (en) * | 2000-04-18 | 2004-09-07 | Sirf Technology, Inc. | Personal communications device with ratio counter |
US6839659B2 (en) * | 2000-06-16 | 2005-01-04 | Isis Innovation Limited | System and method for acquiring data |
US20020169869A1 (en) * | 2001-05-08 | 2002-11-14 | Shugart Technology, Inc. | SAN monitor incorporating a GPS receiver |
US20030103486A1 (en) * | 2001-11-30 | 2003-06-05 | Metin Salt | Time synchronization using dynamic thresholds |
US7352715B2 (en) * | 2001-11-30 | 2008-04-01 | Cellnet Innovations, Inc. | Time synchronization using dynamic thresholds |
EP1671231B1 (en) * | 2003-09-23 | 2019-11-06 | Symantec Operating Corporation | Systems and methods for time dependent data storage and recovery |
US20060090092A1 (en) * | 2004-10-25 | 2006-04-27 | Verhulst Anton H | Clock timing adjustment |
US8533515B2 (en) * | 2009-02-18 | 2013-09-10 | Dolby Laboratories Licensing Corporation | Method and system for synchronizing multiple secure clocks using an average adjusted time of the secure clocks if the average adjusted time is within the limit intersection and using a substitute average adjusted time if the averaged adjusted time is outside the limit intersection |
US20110302443A1 (en) * | 2009-02-18 | 2011-12-08 | Dolby Laboratories Licensing Corporation | Method and System for Synchronizing Multiple Secure Clocks |
US8953581B1 (en) * | 2009-05-13 | 2015-02-10 | Dust Networks, Inc. | Timing synchronization for wireless networks |
US8699406B1 (en) | 2009-05-13 | 2014-04-15 | Dust Networks, Inc. | Timing synchronization for wireless networks |
US9955443B2 (en) | 2009-05-13 | 2018-04-24 | Linear Technology Corporation | Timing synchronization for wireless networks |
US8432851B2 (en) * | 2009-09-30 | 2013-04-30 | Huawei Technologies Co., Ltd. | Method, apparatus, and system for time synchronization |
US9007989B2 (en) | 2009-09-30 | 2015-04-14 | Huawei Technologies Co., Ltd. | Method, apparatus, and system for time synchronization |
US20110075685A1 (en) * | 2009-09-30 | 2011-03-31 | Huawei Technologies Co., Ltd. | Method, apparatus, and system for time synchronization |
US9209402B2 (en) | 2013-04-22 | 2015-12-08 | Joled Inc. | Method of manufacturing EL display device |
US20150215031A1 (en) * | 2013-12-13 | 2015-07-30 | Vt Idirect, Inc. | Time synchronization in a satellite network |
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